Not Applicable
Not Applicable
1. Field of the Invention
This invention pertains generally to semiconductor bonding techniques, and more particularly to a low temperature, insitu, plasma activated wafer bonding apparatus and method.
2. Description of the Background Art
It is well known that direct wafer bonding is an alternative to using organic or inorganic bonding agents for bonding silicon and a number of other semiconductor materials. For example, direct bonding can be facilitated by first activating the surface of the wafer with a base bath (NH4OH:H2O2:H2:O, 1:1:5) for silicon and its oxides, or with an acid bath (HC1:H2O2:H2:O, 1:1:6) or HF dip for nitrides such as AIN and Si3N4. Plasma exposure is another known technique for activating the surfaces of wafers to be bonded. These surface activation methods render the wafer surfaces hydrophilic and amenable to bonding. After surface activation, the wafers are placed in a spinner where they are rinsed in de-ionized water and spun dry. After this step the wafers are placed surface to surface at which point van der Waals forces pull the two wafers into contact.
The contact bonds which are formed in accordance with conventional wet surface activation are generally weak (less than 0.1 MPa), and not suitable for device processing. This is because the process of oxidation (or corrosion of any kind) upon which high temperature direct bonding of semiconductor materials is based is the result of a two step process: migration of the reacting specie(s) to the reaction site, and then the chemical reaction itself. For example, the high temperature oxidation of silicon (T greater than 700xc2x0 C.) is known to follow linear kinetics initially until the oxide thickness becomes so thick that the atomic transport is the limiting process. In other words, initially Si and O atoms are directly adjacent or are very close. All that is required is the transfer of electrons between the atoms for the reaction to occur. However, as the oxide thickness increases, oxygen atoms must migrate to the unreacted silicon through the oxide layer.
The energy that must be supplied to the xe2x80x9csystemxe2x80x9d to cause the Si and the oxygen to migrate and react is quite large and, as such, this particular reaction is not self sustaining at low temperatures. Therefore, the bonds are typically strengthened by high temperature anneals (T greater than 900xc2x0 C.) for silicon and its oxides, and moderate temperature anneals (Txcx9c300xc2x0 C.) for nitrides. Following the anneals the interfacial bond obtains strengths greater than 1-2 MPa up to a maximum of about 4 Mpa (absolute values depending on test method). This strength is sufficient for further processing such as backthinning, polishing, and micromachining, and the interface is generally free from detectable voids. However, the temperatures required for the annealing step have limited the use of conventional direct bonding techniques to applications wherein the materials to be bonded can withstand the high temperature anneal. Unfortunately, the elevated temperature exposure can have a detrimental effect on implanted or diffused etchstop layers via diffusive broadening.
Therefore, while it is known that wafers can be direct bonded, conventional bonding methods are only effective with high temperature anneals and, further, some materials are unable to withstand such high temperatures. Accordingly, high temperature bonding is limited in its application.
To avoid material damage and problems with thermal mismatching in bonding dissimilar materials, there exists a need for a direct bonding process whereby direct bonding can be effected using a low temperature anneal. In addition, to prevent absorption of water and other contaminates present in air, there exists a need for a process to bond wafers to one another without exposing the wafers to wet environments. The present invention satisfies those needs, as well as others, and overcomes the deficiencies inherent in conventional direct bonding techniques.
The present invention pertains to an apparatus and method for directly bonding materials to one another in a dry environment. The invention bonds materials while inside a plasma environment without breaking vacuum or exposing the materials to external environments. In accordance with an aspect of the invention, materials are bonded inside a plasma chamber, prior to exposure to an external environment; that is, materials are bonded insitu. In accordance with another aspect of the invention, a plasma chamber apparatus is provided which can be used for insitu plasma bonding of materials.
The method and apparatus of the present invention are unique because they provide for a completely dry bonding process. This provides for full strength bonding upon contact in most cases, allowing for heterogeneous materials to be bonded. It eliminates any water or other contamination from adsorbing on the surfaces and becoming trapped at the interface, and thus requiring a high temperature annealing step to remove this interface contamination layer via diffusional processes. Some materials systems may require post bond anneals at low temperatures to fully complete the chemical bond reactions.
The present invention provides for bonding wafers without the need for high temperature anneals or use of organic or inorganic bonding agents. The resulting bonded material is free from macroscopic and microscopic voids, and has a strength equivalent to Sixe2x80x94Si bonded materials which have been bonded with the conventional base bath method and annealed at temperatures greater than 900xc2x0 C. The bonded materials can be ground and chemically backthinned, and can be used for Bond and Etchback Silicon on Insulator (BESOI), Smart Cut(copyright) wafers, high voltage and high current devices, radiation resistant devices, micromachined sensors and actuators, and hybrid semiconductor applications.
An object of the invention is to activate the surfaces of materials for direct bonding in a dry environment.
Another object of the invention is to activate the surfaces of materials for room temperature high strength bonds.
Another object of the invention is to eliminate the need for wet chemical treatments for bonding.
Another object of the invention is to provide for precision alignment of micromachined wafer features.
Another object of the invention is to create hydrophilic surfaces on materials to be bonded.
Another object of the invention is to eliminate contamination problems inherent in bonding in wet environments.
Another object of the invention is to eliminate the high temperature annealing step used in conventional direct bonding which is incompatible with many applications (diffused regions) and many materials (GaAs phase separation).
Another object of the invention is to provide for bonding heterogeneous materials.
Another object of the invention is to provide for direct bonding of similar or dissimilar materials.
Another object of the invention is to provide for integration of electronic circuitry into microsensors.
Another object of the invention is to provide for integration of electronic circuitry into optoelectronic devices.
Another object of the invention is to provide for iterative fabrication of multilayered devices.
Another object of the invention is to provide for direct bonding of semiconductor chips.
Another object of the invention is to provide for direct bonded packaging of semiconductor chips.
Another object of the invention is to provide for bonding of wafers having surfaces which are too rough for bonding with conventional bonding methods.
Another object of the invention is to provide for bonding of wafers after failure of conventional bonding methods.
Another object of the invention is to initially bond materials at room temperature and anneal the bonds at low temperatures.
Another object of the invention is to provide for direct bonding at temperatures of approximately 300xc2x0 C. or lower.
Another object of the invention is to allow for bond interface chemistry tailoring (e.g. termination species can be changed by specific plasma gas without altering the basic process).
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.